This invention relates to a process for the production of synthesis gas from a hydrocarbon feed.
More specifically it concerns a novel favourable combination of reforming technologies involving catalytic partial oxidation which allows for the preparation of synthesis gas from a hydrocarbon stream with process improvements. The hydrocarbon stream such as methane, natural gas, butane, LPG or naphtha may further contain other components such as H2, CO, CO2, N2, NH3, HCN, inorganic or organic sulphur compounds.
Amongst the known reforming technologies are gas phase partial oxidation (GPOX), various forms of steam reforming e.g. methane steam reforming (MSR), autothermal reforming (ATR) and catalytic partial oxidation (CPO). The ATR process in fact involves a sequential combination of GPOX and MSR, the CPO being a process wherein the reactions of GPOX are avoided in the gas phase and instead performed on the surface of the highly active catalyst by which the steam reforming reactions (incl. MSR) are conducted simultaneously.
The catalytic partial oxidation reactor comprises a mixing zone, where the hydrocarbon feedstock and steam admixture is further mixed with the oxidant and a catalytic zone where the feedstock admixture is reacted according to the main reaction schemes (hydrocarbons represented as methane in the schemes):
xe2x80x83CH4+xc2xdO2xe2x86x92CO+2H2
CH4+2O2xe2x86x92CO2+2H2O
2H2+O2xe2x86x922H2O
CH4+H2OCO+3H2(MSR)
CO+H2OCO2+H2(shift)
All higher hydrocarbons are converted in the CPO. The exit gas from the CPO reactor contains hydrogen, carbon monoxide, carbon dioxide, methane and steam in amounts such that the MSR and shift reactions are close to equilibrium. Thus, basically the same reactions occur in a CPO reactor as in an ATR reactor, only the reactions in a CPO reactor occur simultaneously and catalytically over the catalyst bed.
Characteristic of CPO is that it may be run soot-free at a low steam to carbon ratio and in some cases even without steam addition.
In order to avoid the risk of flames, the inlet temperature to the CPO reactor must be rather low compared to the inlet temperatures to an ATR. The maximum preheat temperature depends among other parameters on the actual oxygen to hydrocarbon ratio (as molar ratio of O2/total content of carbon from hydrocarbons), and the steam to carbon ratio (molar ratio of H2O/total content of carbon from hydrocarbons). The molar content of carbon is calculated as the molar content of the hydrocarbon times the carbon contents of the hydrocarbon.
A high conversion of the hydrocarbon (low methane leakage) is only attained if the temperature is raised sufficiently through the reactor.
The need for a high exit temperature implies the requirement of a high oxygen to carbon ratio of the feed streams, typically 0.5-0.90, which disadvantageously leads to a high consumption of oxidant and thus a high production cost.
According to U.S. Pat. No. 5,883,138 gas phase oxidation reactions can be avoided even with premixed feed at high temperature, if the gas mixture in the mixing zone is passed before a maximum residence time (most preferably less than 0.5 milliseconds) and at a high velocity (most preferably between 50 and 300 ft/sec) into a CPO reaction zone. The methods of a reduced delay time are insecure and running the CPO process according to this principle represents a major risk of explosion.
The catalyst in a CPO reactor often contains one or more noble metals (e.g. Rh, Ir, Pt, Pd). The catalyst may comprise nickel and cobalt as catalytic components for some CPO uses, and may further comprise noble metals. Another drawback in the CPO reforming technology is that when the oxygen to carbon ratio is high, the temperature on surface of the catalyst becomes as high as 1100-1300xc2x0 C., which leads to the detrimental sintering of the catalyst.
Thus, a general object of this invention is to provide a CPO process being operated at low oxygen consumption and without the safety risks of running the process under conditions where the premixture is explosive.
As known in the autothermal reforming, combustion of hydrocarbon feed mixed with steam is carried out with substoichiometric amounts of oxygen by flame reactions in a burner combustion zone. The oxygen source may be any oxygen containing stream (hereinafter oxidant) such as pure oxygen, air or mixtures thereof. If the hydrocarbons are represented by methane, the reaction schemes are:
CH4+xc2xdO2xe2x86x92CO+2H2
CH4+2O2xe2x86x92CO2+2H2O
2H2+O2xe2x86x922H2O
Subsequently, the partially combusted hydrocarbon stream is steam reformed in a fixed bed of steam reforming catalyst, according to the following reaction schemes.
CH4+H2OCO+3H2(MSR)
CO+H2OCO2+H2(Shift)
Substoichiometric combustion of hydrocarbons may disadvantageously lead to the formation of soot. Soot formation may though be avoided by using a specific burner design and through controlling the operating conditions.
E.g. soot can be avoided if the amount of steam relative to the other components send to the autothermal reformer is above a critical value. The limiting amount of steam can be expressed as the critical steam to carbon ratio, which is the molar feed flow rate of steam to the molar flow rate of carbon in the hydrocarbon feed.
Examples of operation conditions, which do not result in soot formation, are summarised in the patent application EP 936,183 Peter Seier Christensen et al. Further, it was found by the inventors that a hydrocarbon feed can be reformed soot free in an autothermal reformer at a given temperature and a given steam to carbon ratio if the operating pressure is increased above a critical value.
It is also known from the U.S. Pat. No. 5,628,931 that the autothermal process can be run soot-free at a low steam to carbon ratio or even in the absence of steam, wherein the reformed product stream has a temperature in the range from 1100xc2x0 C.-1300xc2x0 C.
The amount of steam added to the ATR process and the temperature of the reformed product, which exits the autothermal reformer, are some of the main factors which decide the composition of the synthesis gas obtained by the autothermal reforming process. Typically, the exit temperature is above 700xc2x0 C., e.g. 900-1100xc2x0 C., and the oxygen content in the oxidant stream relative to the carbon content in the hydrocarbon feedstock is between 0.4 and 0.9.
In many cases the amount of steam required in order to eliminate the risk of soot formation is higher than the amount of water desired as to achieving a specific synthesis gas composition down stream the autothermal reformer. Such cases are e.g. synthesis gas applications related to methanol or methanol derivatives, Fischer-Tropsch process, olefins, alcohols and aldehydes.
A low steam to carbon ratio is typically requested to decrease the process costs.
However, it may not be attractive to increase the pressure or the exit temperature of the process in order to eliminate soot formation.
An increment of the operating pressure inherently leads to an increased concentration of methane in the product gas, especially at low steam to carbon ratio. Further, it involves the need for further compression of the oxygen source and of the availability of process steam at a higher pressure. A higher pressure of the synthesis gas delivered by the ATR process should preferably be a benefit to the downstream process, which receives the synthesis gas, in order to justify the choice of a higher operating pressure of the ATR. In parallel the increase of the methane concentration in the synthesis gas should not constitute too much of a drawback to the down-stream process.
The temperature of the product gas is a function of the preheating temperatures of the individual feed streams, the individual amounts of these streams and the degree of conversion. There are practical limits to the preheating temperatures of the feeds, and the current reactions are literally brought into equilibrium, meaning that if the temperature of the reformed product is to be as high as 1100xc2x0 C.-1300xc2x0 C., the oxygen expenses are disadvantageously increased.
Thus, another general object of this invention is to provide an autothermal reforming process, which can be operated soot-free at low/moderate pressures, without the increased need of steam or oxygen.
In general in partial oxidation processes there is a wish to obtain certain product gas compositions useful for a given backend product, e.g. carbon monoxide or hydrogen, or process, e.g. Fischer-Tropsch, methanol or DME synthesis, without having by-products such as soot. The group of partial oxidation processes comprises ATR, GPOX and partial oxidation by means of a membrane, in which the oxidant is supplied through a membrane to the reaction zone. For all partial oxidation processes the parameters determining the product composition are the reactor pressure, the steam to carbon ratio and the product exit temperature controlled by the inlet temperature of the individual feeds and the amount of oxidant added.
It has now been found that when partly or fully processing a hydrocarbon feedstock in a catalytic partial oxidation (CPO) reactor, the partially oxidised (prereformed) hydrocarbon feedstock may then be reformed at improved conditions in a subsequent reforming step. E.g. the partially oxidised hydrocarbon can be processed soot free in an autothermal reformer at a lower steam to carbon ratio than normally applied. Without wishing to be bound to any theory the inventors suggest that the soot formation in the ATR depends on the content of hydrocarbons in the feedstock especially higher hydrocarbons, in particular olefins, whereas carbon monoxide and carbon dioxide will not form soot. Having the feed processed in a CPO reactor the composition of the gas is converted to CO, CO2, H2, H2O and CH4. This feedstock will have a much lower critical soot value than the original feedstock, and therefore it is possible to lower the steam to carbon ratio or to operate at a lower pressure at a constant steam to carbon ratio.
When the CPO reactor is used in combination with another reforming technology in the present invention, new operating conditions are advantageously applied to the CPO. As further an advantage, the oxygen consumption can be reduced, as the methane leakage exit the CPO is allowed to be high, the methane being converted in the reforming process following the CPO. With the reduction in oxygen is obtained the reduced sintering of the catalyst and the reduced risk of detonation/flames in the inlet zone of the CPO reactor.
With the invention is provided, i.e. with the novel combination of the CPO reactor and a partial oxidation reactor, a kind of soot guard of the subsequent partial oxidation reforming technology in combination with the CPO reactor. The synthesis gas exiting the partial oxidation reactor of the present invention is soot-free, however, similar in composition to a synthesis gas at the exit of a conventional partial oxidation reactor fed directly with the hydrocarbon feed at conditions which lead to soot formation.
Yet another advantage of the present invention is that the preheat temperatures of the individual streams of hydrocarbon and oxidant are considerably lower than what is typically applied for partial oxidation reactor alone. The lower preheat temperatures lead to a reduced need of heating duty, the allowance of a lower temperature level of the heating medium, and lower demands on the construction materials for the respective heaters, and for oxygen services, lower risk of rupture.
In another aspect of the present invention it provides for a postconverter (partial oxidation reactor) of a CPO, eliminating current technological problems with CPO, as mentioned.
This aspect of the invention is fulfilled as long as, given soot free operation, the ratio of the oxygen/hydrocarbon at the inlet to the CPO reactor in the combined layout is less than in the inlet to a CPO based plant; i.e:
(O2/C)CPO COMBI INLET less than (O2/C)CPO INLET